Aqueous fluid sets for inkjet printing methods

ABSTRACT

wherein R is a substituted or unsubstituted phenyl group or a substituted or unsubstituted phenoxy group, in a total amount of at least 0.5 weight % and up to and including 2 weight %, based on the total weight of the aqueous inkjet ink composition. Such aqueous particle-free fluids can be used as replenishment or printhead maintenance fluids. Two or more of such aqueous particle-free fluids can be packaged, sold, and use as part of fluid sets.

REFERENCED APPLICATIONS

This is a divisional application of U.S. application Ser. No. 16/171,446 (recently allowed).

Reference is made to the following patent applications, the disclosures of all of which are incorporated herein by reference:

U.S. Ser. No. 16/171,432, filed on Oct. 26, 2018, by Sowinski, Bugner, and Cook, and entitled “Aqueous Inkjet Ink and Ink Sets”;

U.S. Ser. No. 16/171,455, now U.S. Pat. No. 10,569,587, by Sowinski et al.; and

U.S. Ser. No. 16/171,433, filed on Oct. 26, 2018, by Maldonado, Sowinski, Cook, and Bugner, and entitled “Absorbent Article with Graphics Printed in Preservative-free Ink, and Methods of Manufacture Thereof.”

FIELD OF THE INVENTION

This invention relates to fluid sets that comprise two or more aqueous particle-free fluids that are useful in inkjet printing methods either as printhead maintenance fluids or as ink replenishment fluids. Such fluids are particle-free, that is free of pigment colorants, but contain a unique class of preservatives that inhibit the growth of microorganisms.

BACKGROUND OF THE INVENTION

The inkjet printing of substrates intended for direct contact with food or skin requires careful consideration of the toxicity and the potential for unintended side effects from human exposure to the chemicals contained in the inkjet inks. The most toxic chemicals in aqueous inkjet ink compositions tend to be the industrial biocides used to prevent microbial growth of bacteria, yeasts, and molds to extend composition shelf-life. Isothiazolinones and formaldehyde released from precursors or donors, for example, are generally safe and have high antimicrobial efficacy within their prescribed usage levels. Nonetheless, such compounds have been recently found objectionable in certain products intended for skin contact because sensitive users can experience skin irritation and dermatitis following exposure to even trace levels. Regulatory bodies and advocacy groups are becoming more influential in what chemicals are used in such consumer products. In addition, the population of consumers is becoming more aware of the use of common preservatives in various products and are developing a preference for products that do not contain them.

A need remains to provide alternative preservatives (or biostats and biocides) with a record of safety and acceptance in skin contact applications that are effective at suppressing microbial contaminant growth in aqueous inkjet ink compositions without destabilizing pigment or dye-based colorants or compromising the high-quality printing process in some other fashion.

Piezoelectric drop-on-demand (DOD) inkjet inks are disclosed in U.S. Patent Application Publication 2017/0073535A1 (Kusukame et al.) using selections of conventional and unconventional components.

The use of 2-phenoxyethanol (identified as ethylene glycol phenyl ether) in continuous inkjet printing (CIJ) processes using aqueous dye-based and pigment-based inks is disclosed in U.S. Pat. No. 6,071,334 (Wider et al.).

Ethylene glycol phenyl ether is described as a substantially water insoluble oil at about 3 weight % in thermal DOD black inkjet inks comprised of self-dispersed carbon black pigments, as disclosed in U.S. Pat. No. 5,749,952 (Tsang et al.)

Pigmented thermal DOD inkjet ink compositions using self-dispersed black pigment and acrylate polymer binder experience reduced “deceleration” (loss of drop velocity during periods of extended nozzle firing) if a deceleration-alleviating component selected from a group of materials including ethylene glycol phenyl ether is incorporated in the ink as described in U.S. Pat. No. 6,726,757B2 (Sarkisian et al.).

The use of 2-phenylethanol (identified as β-phenylethyl alcohol) in a fragrant aqueous inkjet ink composition intended to impart rose scent is disclosed in U.S. Pat. No. 6,123,757 (Yang et al.).

Despite these various innovations, there is an unmet need for aqueous inkjet ink compositions and particle-free fluids that are substantially free of common preservatives or biocides, while preserving desired continuous inkjet printing (CIJ) features and high-quality images.

SUMMARY OF THE INVENTION

The present invention provides a fluid set comprising two or more aqueous particle-free fluids, each of which independently has a dynamic viscosity of less than or equal to 5 centipoise (5 mPa-sec) at 25° C., and each of which two or more aqueous particle-free fluids independently consists essentially of:

(b) a composition consisting of one or more compounds represented by the following Structure (I):

HO—CH₂—CH₂—R  (I)

wherein R is a substituted or unsubstituted phenyl group or a substituted or unsubstituted phenoxy group, in a total amount of at least 0.5 weight % and up to and including 2 weight %, based on the total weight of the aqueous inkjet ink composition.

The stable aqueous pigment dispersions, inkjet ink compositions, and aqueous pigment free fluids according to the present invention exhibit improved resistance to microbial growth and are acceptable for skin contact because of the presence of 2-phenoxyethanol, 2-phenylethanol, or a combination thereof when commonly used industrial biocidic preservatives such as isothiazolinone derivatives and formaldehyde releasers are withheld from such formulations. Used individually or in combination, these ethanol derivatives were effective at suppressing the growth of a representative mixture of gram-negative and gram-positive bacteria during in vitro challenge testing of the inkjet ink compositions. Combined with appropriate supplemental antimicrobial compounds, susceptibility to growth of a very broad array of microbial organisms in the compositions can be satisfactorily minimized during extended challenge testing. A method of continuous inkjet (CIJ) printing is provided according to the present invention when the aqueous inkjet ink compositions are used and replenished in the CIJ printer fluid system for recirculating ink using an aqueous particle-free fluid containing 2-phenoxyethanol, 2-phenylethanol, or other alternative preservatives. The printed substrates obtained according to the present invention can be porous matted media (for example, paper), woven media (for example, a textile), and porous or non-porous films (for example, a polymer sheet) that can be pre-treated or pre-coated to better receive and bind the printed inkjet inks to the printed media or article.

Further details and advantages of the present invention can be understood from the teaching provided below.

DETAILED DESCRIPTION OF THE INVENTION

The following discussion is directed to various embodiments of the present invention and while some embodiments can be desirable for specific uses, the disclosed embodiments should not be interpreted or otherwise considered to limit the scope of the present invention, as claimed below. In addition, one skilled in the art will understand that the following disclosure has broader application than is explicitly described in the discussion of any embodiment.

Definitions

As used herein to define various components of the aqueous organic pigment dispersions, aqueous inkjet ink compositions, and other materials used in the practice of this invention, unless otherwise indicated, the singular forms “a,” “an,” and “the” are intended to include one or more of the components (that is, including plurality referents).

Each term that is not explicitly defined in the present application is to be understood to have a meaning that is commonly accepted by those skilled in the art. If the construction of a term would render it meaningless or essentially meaningless in its context, the term should be interpreted to have a standard dictionary meaning.

The use of numerical values in the various ranges specified herein, unless otherwise expressly indicated otherwise, are considered to be approximations as though the minimum and maximum values within the stated ranges were both preceded by the word “about.” In this manner, slight variations above and below the stated ranges may be useful to achieve substantially the same results as the values within the ranges. In addition, the disclosure of these ranges is intended as a continuous range including every value between the minimum and maximum values as well as the end points of the ranges.

As used herein, the parameter “acid number” (also known as acid value) is defined as the milligrams (mg) of potassium hydroxide required to neutralize 1 g of an acidic polymer.

The term “aqueous” in aqueous organic pigment dispersions, and aqueous inkjet ink compositions, according to the present invention means that the water content is greater than 60 weight % based on the total amount of solvents. Thus, water is the predominant solvent in the aqueous medium.

Ink and fluid dynamic viscosity can be measured by any of well-known techniques. Preferred methods include measurement of the timing of mass flow through a capillary as in a capillary viscometer, or measurement of ball drop velocity through a fluid, using for example a rolling ball viscometer. Both a capillary flow viscometer and a commercially available Anton Paar Automated MicroViscometer (AMVn) employing the rolling ball technique can be used to measure the dynamic viscosities reported herein. All ink dynamic viscosity values disclosed herein were measured under gravity induced shear at approximately 24° C. to 26° C. It will be appreciated that the values cited are reported as centipoise (cP) or milliPascal seconds (mPa-sec) and that 1 cP=10⁻³ Pascal-seconds (Pa-s)=10⁻² dyne-s/cm². While viscosities can be measured with high precision, viscosity values here are reported to one or two decimal places only, and they are normally rounded values and not truncated values. All claims reciting ink viscosities are intended to be interpreted in terms of values in mPa-sec normally rounded to one decimal point. Thus, the various aqueous inkjet ink compositions can have a viscosity of up to and including 10 centipoise (10 mPa-sec).

The Wilhelmy plate method is a well-known technique for measuring the static surface tension of a liquid ink or service fluid at a solid interface. The technique involves a plate of known dimensions, typically selected from a roughened platinum alloy, suspended from a balance. The plate is contacted with a solution of interest and a vertical force is applied to the plate to form a liquid meniscus between the solution and plate. The resulting surface tension is given according to equation (1):

σ=F/L cos(θ)  (1)

where σ is the surface tension of the liquid, F is the force acting on the balance (milli-Newtons/meter), L is the wetted length of the plate in millimeters, and θ is the contact angle between the plate and solution.

Typically, the roughened platinum results in a contact angle very close to zero and the cosine of θ goes to 1. A complete theoretical treatment of the method can be found in, for example, “A Method for Determining Surface and Interfacial Tension Using a Wilhelmy Plate,” Colloid and Polymer Science, 255 (7), pages 675-681. A number of commercially available instruments are known for measuring surface tension, however, the instrument used to report surface tension values in the present invention is a Krüss Model K10ST tensiometer.

The pigment colorants used in the various embodiments of the present invention are not generally self-dispersing meaning that they require the presence of one or more organic polymeric pigment dispersants bound to some fraction of the surface of the pigment particles to keep them suspended in an aqueous medium.

The terms “water-soluble” and “aqueous-soluble” mean that 1 mass part of solute material can be dissolved in as little as less than 1 mass part (i.e., more soluble solute) and in as much as 1,000 mass parts (that is, less soluble solute) of distilled water at 25° C. to provide a homogeneous and visibly clear solution.

The term “solvo-surfactant” refers to a compound or combination of compounds that are effective solvents for dried “ink,” and are volatile solvents having a boiling point at sea level of less than 200° C. and capable of reducing fluid surface tension and self-aggregating. Further details of such compounds are provided below.

For clarification of definitions for any terms relating to polymers, reference should be made to “Glossary of Basic Terms in Polymer Science” as published by the International Union of Pure and Applied Chemistry (“IUPAC”), Pure Appl. Chem. 68, 2287-2311 (1996). However, any definitions explicitly set forth herein should be regarded as controlling. Polymers can be prepared from ethylenically unsaturated polymerizable monomers using free radical polymerization or acid catalyzed polymerization processes, or by reaction of appropriate condensation monomers (for example diols and diisocyanates) using known condensation polymerization processes.

Unless otherwise indicated, the terms “polymer” and “polymeric” refer to both homopolymers and copolymers, each having a stated weight distribution average molecular weight (M_(w)) or a number distribution average molecular weight (M_(n)) as measured using gel permeation chromatography (polystyrene standard).

The term “copolymer” refers to polymers that are derived from two or more different monomers, in random order or a predetermined order (for example, block) along the polymer backbone. That is, each copolymer comprises at least two recurring units having different chemical structures.

The term “backbone” refers to the chain of atoms in a polymer to which a plurality of pendant groups can be attached. An example of such a backbone is an “all carbon” backbone obtained from the polymerization of one or more ethylenically unsaturated polymerizable monomers. However, other backbones can include heteroatoms wherein the polymer is formed by a condensation reaction or some other means.

Aqueous Organic Pigment Dispersions

(a) Polymer dispersed pigment colorants useful in the practice of this invention can be used singly, or in combinations of two or more different polymer-dispersed pigment colorants, to provide any desired color or hue. For example, a polymer-dispersed carbon black pigment can be combined with a different polymer-dispersed colored organic pigment in the same aqueous pigment dispersion or aqueous inkjet ink composition. The exact choice of polymer-dispersed pigment colorants will depend upon the specific application, performance, color reproduction, and image stability that are desired. Useful polymer-dispersed pigment colorants are described for example in U.S. Pat. No. 5,026,427 (Mitchell et al.), U.S. Pat. No. 5,141,556 (Matrick), U.S. Pat. No. 5,160,370 (Suga et al.), and U.S. Pat. No. 5,169,436 (Matrick), the disclosures of all of which are incorporated herein by reference.

Useful pigment colorants that can be dispersed with organic polymers include but are not limited to, azo pigments, monoazo pigments, disazo pigments, azo pigment lakes, β-naphthol pigments, naphthol AS pigments, benzimidazolone pigments, disazo condensation pigments, metal complex pigments, isoindolinone and isoindoline pigments, quinacridone pigments, polycyclic pigments, phthalocyanine pigments, perylene and perinone pigments, thioindigo pigments, anthrapyrimidone pigments, flavanthrone pigments, anthanthrone pigments, dioxazine pigments, triarylcarbonium pigments, quinophthalone pigments, diketopyrrolo pyrrole pigments, titanium dioxide, iron oxide, and carbon blacks. Representative useful yellow, black, green, brown, red, magenta, cyan, blue, orange, and violet pigment colorants are described in Col. 7 (line 48) to Col. 8 (line 5) of U.S. Pat. No. 8,173,215 (Sowinski et al.), the disclosure of which is incorporated herein by reference.

Useful pigment colorants can be accompanied or dispersed using suitable polymer dispersants that are well known in the art (as cited above). Representative useful organic polymer dispersants can be prepared from at least one anionic hydrophilic monomer such as an acrylic acid or methacrylic acid monomer, or combinations thereof, and for example, at least one monomer comprised of a hydrophobic methacrylate or acrylate monomer having an aliphatic chain having 12 or more carbon atoms, as described for example in U.S. Patent Application Publication 2007/0043144 (House et al.), the disclosure of which is incorporated herein by reference. Further details of useful organic polymer dispersants, including useful monomer recurring units, monomer amounts, and M_(w), are provided in Col. 5 (line 45) to Col. 6 (line 31) of U.S. Pat. No. 8,173,215 (noted above).

Many useful organic polymer dispersants are anionic acrylic polymers formed from at least one anionic hydrophilic monomer described above having a weight average (M_(w)) molecular weight of at least 500 Daltons but less than 100,000 Daltons, and more likely up to and including 15,000 Daltons, or up to and including 10,000 Daltons.

One or more organic polymer dispersants for the pigment colorants can be present in an amount that would be readily apparent to one skilled in the art depending upon the aqueous medium, the chosen organic pigments, and other components of the aqueous inkjet ink composition.

In addition to the polymeric dispersants, nonionic or anionic surfactants can be present with the pigment colorants as is known in the art. Representative materials of this type include but are not limited to, sodium dodecylsulfate or sodium oleylmethyltaurate as described for example in Col. 7 (lines 15-23) of U.S. Pat. No. 8,173,215 (noted above).

Particle sizes for each useful polymer-dispersed pigment colorants are as follows.

Particle size for the various polymer-dispersed pigment colorants refers to the approximate diameter of a generally spherical pigment particle or to the approximate largest characteristic dimension of a non-spherical particle. More particularly, the diameter of a sphere having the same volume as the particle is determined in practice (that is, equivalent sphere diameter). The desired median particle diameter (or 50^(th) percentile particle diameter) of each of the pigmented colorants used according to this invention can be less than 300 nm, less than 150 nm, less than 70 nm, or even less than 60 nm, such that 50% of the volume of the particles is composed of particles having diameters smaller than the indicated diameter. In addition, at least 95% of the total primary particles of each polymer-dispersed pigment colorant used according to this invention have a particle diameter of less than 500 nm, less than 300 nm, less than 150 nm, or even less than 110 nm. This refers to the 95^(th) percentile particle diameter that is the classified particle size distribution such that 95% of the volume of organic pigment particles is provided by particles having diameters smaller than the indicated diameter. Particle size (or particle volume) can be readily measured using a conventional dynamic laser light scattering particle size analyzer. Instrumental techniques for analyzing and reporting nanoparticle sizes are usefully described in “Particle Size Characterization,” published by the National Institute of Standards and Technology (“NIST”), Special Publication 960-1, 93-139 (2001).

The (b) composition used in the practice of this invention consists of one or more compounds represented by the following Structure (I):

HO—CH₂—CH₂—R  (I)

wherein R is a substituted or unsubstituted phenyl group or a substituted or unsubstituted phenoxy group. For example, R can be either an unsubstituted phenyl group or an unsubstituted phenoxy group. Mixtures of compounds represented by Structure (I) can also be used.

Thus, either or both of 2-phenoxyethanol and 2-phenylethanol can be used to advantage in the practice of this invention for the advantages described above. 2-Phenoxyethanol is also known as ethylene glycol phenyl ether; ethylene glycol monophenyl ether; and 1-hydroxy-2-phenoxyethane, and can be obtained as DOWANOL PhE, DOWANOL EPh, or DOWANOL EP from various commercial sources. 2-Phenylethanol is also known as phenylethanol; 2-phenylethan-1-ol; phenethyl alcohol; β-hydroxyethylbenzene; phenylethyl alcohol; β-phenylethanol; and benzyl carbinol and can be obtained from various commercial sources.

An aqueous medium such as water and any water-miscible organic solvents, can be present in any desirable amount that would provide desired viscosity and other physical properties for storage or use of an aqueous pigment dispersion to make aqueous inkjet ink compositions.

An aqueous pigment dispersion according to this invention can be prepared by any method commonly used in the art, and typically involves two steps: (a) a dispersing or milling step to break up aggregates of one or more pigment colorants into primary particles wherein a primary particle is defined as the smallest identifiable subdivision in a particulate system; and (b) a dilution step in which the pigment colorant dispersion from step (a) is diluted by addition of aqueous medium and any other additives. Milling details are described for example in Col. 6 (line 58) to Col. 7 (23) of U.S. Pat. No. 8,173,215 (noted above).

Aqueous Inkjet Ink Compositions

The aqueous inkjet ink compositions according to the present invention generally have a dynamic viscosity that is generally less than or equal to 5 centipoise (5 mPa-sec), or at least 1 centipoise (1 mPa-sec) and up to and including 3 centipoise (3 mPa-sec), all measured at 25° C. using a rolling ball viscometer, or a capillary viscometer, and known procedures.

The aqueous inkjet ink compositions also typically have a pH of at least 7.5 and up to and including 11, or more likely of at least 8 and up to and including 9. When the aqueous inkjet ink composition is used in hardware with nickel or nickel-plated apparatus components, a corrosion inhibitor such as the sodium salt of 2- or 5-methyl-1-H-benzotriazole can be added, and the pH can be adjusted to at least 10 and up to and including 11. If printheads fashioned out of silicon are used for inkjet printing, the aqueous inkjet ink composition pH can be adjusted to at least 7.5 and up to and including 10, or at least 8 and up to and including 9.5.

Aqueous inkjet ink compositions according to the present invention comprise one or more (a) polymer-dispersed pigment colorants in a total amount of at least 0.9 weight % and up to and including 6 weight %, or at least 1.5 weight % and up to and including 5 weight %, based on the total weight of the aqueous inkjet ink composition. Useful polymer-dispersed pigment colorants, and useful particle size parameters, are described above. Two or more polymer-dispersed pigment colorants can be used if desired to provide a desired hue or color in the resulting inkjet-printed image.

The (b) composition described above consisting of one or more compounds represented by Structure (I) is present in the aqueous inkjet ink compositions, in a total amount of at least 0.5 weight % and up to and including 2 weight %, or even at least 1 weight % and up to and including 1.8 weight %, based on the total weight of the aqueous inkjet ink composition.

Each aqueous inkjet ink composition according to the present invention comprises (c) one or more compounds selected from water-soluble humectants, co-solvents, and both water-soluble humectants and co-solvents. The co-solvents are generally water-soluble or water-miscible organic solvents having a viscosity that is greater than 1 centipoise (1 mPa-sec) and that can exceed even 40 centipoise (40 mPa-sec) when measured at 25° C. using a standard rolling ball, capillary, or spinning plate viscometer. Any water-soluble humectant or co-solvent known in the inkjet art that is compatible with the other requirements of the invention can be used. While an individual humectant can be employed, mixtures of two or more humectants, each of which imparts a useful property, can be used. Representative humectants are described for example, in U.S. Pat. No. 9,828,513 (Lussier et al.), the disclosure of which is incorporated herein by reference, and include classes of compounds such as (1) mono-alcohols; (2) polyhydric alcohols; (3) lower mono- and di-alkyl ethers derived from a polyhydric alcohol; (4) nitrogen-containing compounds such as urea, 2-pyrrolidone, N-methyl-2-pyrrolidone, and 1,3-dimethyl-2-imidazolidinone; and (5) sulfur-containing compounds such as 2,2′-thiodiethanol, dimethyl sulfoxide, and tetramethylene sulfone. Useful co-solvents are also included within such classes of compounds.

The one or more (c) humectants, co-solvents, or both humectants and co-solvents can be present in an amount of less than 20 weight %, or at least 0.5 weight %, or at least 1 weight % and up to and including 15 weight %, or at least 3 weight % and up to and including 10 weight %, all based on the total weight of the aqueous inkjet ink composition. Highly preferred levels of the one or more (c) humectants, co-solvents, or both humectants and co-solvents are at least 4 weight % and up to 8 weight %.

One or more (d) supplemental antimicrobial agents can also be present in the aqueous inkjet ink compositions, and such materials are different from the (b) composition described above and represented by Structure (I). Representative materials include but are not limited to, iodopropynyl butyl carbamate (CAS 55406-53-6), piroctone olamine (CAS 68890-66-4), 2,4-dichlorobenzyl alcohol (CAS 1777-82-8), boric acid (CAS 10043-35-3) and monovalent and divalent metal ion salts derived from boric acid, and combinations of these materials. A useful amount of the one or more supplemental antimicrobial agents is at least 0.01 weight % and up to and including 3 weight %, based on the total weight of the aqueous inkjet ink composition.

Each aqueous inkjet ink composition according to the present invention can further comprise one or more anionic polyurethanes, each having an acid number of at least 50, or of at least 60 and up to and including 150, or even at least 55 and up to and including 90, which acidic polymers are described in more detail below.

Alternatively, or in addition to the anionic polyurethanes, the aqueous inkjet ink composition can comprise one or more anionic non-aromatic acrylic or anionic styrene-acrylic polymers, each having an acid number of at least 50, or of at least 120 and up to and including 240, or even at least 160 and up to and including 220, which acidic polymers are described in more detail below. Mixtures of two or more of such materials can be used if desired.

Representative examples of both types of anionic polymers are described for example in U.S. Pat. No. 8,430,492 (Falkner et al.) the disclosure of which is incorporated herein by reference, and in U.S. Pat. No. 9,783,553 (noted above). Particularly useful anionic polyurethanes contain a polyether diol unit and can be identified as polyether polyurethanes. Such anionic polyether polyurethanes generally can have a molecular weight (M_(w)) of at least 10,000 Daltons and up to and including 30,000 Daltons or at least 15,000 Daltons and up to and including 25,000 Daltons. For example, particularly useful polyether polyurethanes are individually represented by Structure (I) in U.S. Pat. No. 9,783,553 (noted above).

Useful water-soluble or water-dispersible anionic polyether polyurethanes can be prepared as described for example in [0045]-[0049] of U.S. Patent Application Publication 2008/0207811 (Brust et al.), the disclosure of which is incorporated herein by reference. The acidic groups in the anionic polyether polyurethanes can be at least partially and up to 100% neutralized (converted into salts) using monovalent inorganic bases such as alkaline metal hydroxides or organic amines such as dimethylethanolamine.

Representative anionic non-aromatic acrylic polymers and anionic styrene-acrylic polymers useful in the present invention are also described for example in [0061] of U.S. Patent Application Publication 2008/207811 (noted above). Examples of useful anionic styrene-acrylic polymers include those commercially available under the trademarks JONCRYL® (BASF Corp.), TRUDOT® (formerly available from Mead Westvaco Co.), and VANCRYL® (Allnex USA, Inc.).

The useful amounts of such anionic polymers are readily known in the art and can be up to and including 15 weight %, or up to and including 10 weight %, all based on the total weight of the aqueous inkjet ink composition. Particularly useful amounts of anionic polymers range are at least 1 weight % and up to and including 5 weight %, including anionic non-aromatic acrylic polymers and anionic styrene-acrylic polymers used as the pigment dispersant.

In addition, modified polysiloxanes can be present in the aqueous inkjet inks compositions. Examples of such materials include ethoxylated or propoxylated silicone-based “surfactants” that can be obtained commercially under the trademarks SILWET® (CL Witco), and BYK® (Byk Chemie) such as BYK® 348 and 381, as well as Dow Corning DC67, DC57, DC28, DC500W, and DC51. Non-silicone surfactants can also be used, including but not limited to anionic, cationic, nonionic, or amphoteric surfactants such as those commercially available as SURFYNOL® surfactants (Evonik Corp.) including SURFYNOL® 440 and 465 alkynediol surfactants. Useful amounts of such materials are readily apparent to one skilled in the art. Particularly useful amounts of selected surfactants are described in U.S. Pat. No. 8,455,570B2 (Lindstrom et al.), the disclosure of which is herein incorporated by reference.

It can be useful to include one or more “promoters” in the aqueous inkjet ink compositions potentially to enhance the effectiveness of the compounds represented by Structure (I) in the (b) composition. Such materials are generally alkane diols, each having at least 7 carbon atoms and up to and including 12 carbon atoms, and particularly having at least 7 carbon atoms and up to and including 10 carbon atoms. Representative useful compounds that can be used singly or in combination as promoters include but are not limited to, 1,7-heptanediol, 1,2-heptandiol, 2-ethyl-1,3-hexanediol, 1,2-octandiol, 3,6-octanediol, 2,2,4-trimethyl-1,3-pentandiol, 1,2-nonanediol, 1,10-decanediol, 1,12-dodecanediol, and others that would be readily apparent to one skilled in the art. 1,2-Octanediol is particularly useful in this regard.

One or more promoters can be present in the aqueous inkjet ink compositions at a total amount of less than or equal to 1.5 weight % or less than or equal to 1.25 weight %, based on the total weight of the aqueous inkjet ink composition. A minimum amount can be at least 0.3 weight %.

Colorless fluorescent colorants (dyes or pigments) can also be present in the aqueous inkjet ink compositions in amounts readily apparent to one skilled in the art, and examples of such compounds are described in U.S. Patent Application Publication 2014/231674 (Cook), the disclosure of which is incorporated herein by reference.

Other additives that can be present in the aqueous inkjet ink compositions, in amounts that would be readily apparent to one skilled in the art, include but are not limited to: surfactants besides those described above to adjust composition surface tension, including but not limited to the TERGITOL® 15-S and TMN series nonionic surfactants, BRIJ® series nonionic surfactants; TRITON® series nonionic surfactants, ZONYL® fluoro surfactants; PLURONIC® nonionic surfactants; TETRONIC® nonionic surfactants, SILWET® nonionic surfactants, and SURFYNOL® nonionic surfactants, and various anionic and cationic surfactants mentioned in Col. 10, line 64 to Col. 11, line 14 of U.S. Pat. No. 8,173,215 (noted above)]; thickeners; conductivity-enhancing agents; drying agents; waterfast agents; viscosity modifiers; pH buffers; antifoamants; wetting agents; corrosion inhibitors; antifoamants and defoamers (such as SURFYNOL® DF-110L, PC, MD-20, and DF-70); UV radiation absorbers; antioxidants; and light stabilizers available under the trademarks TINUVIN® (BASF Corp.) and IRGANOX® (BASF Corp.), as well as other additives described in Col. 17 (lines 11-36) of U.S. Pat. No. 8,455,570 (noted above). The useful amounts of such materials would be readily apparent to one skilled in the art using routine experimentation.

Water is generally present in each aqueous inkjet ink as the primary aqueous medium, in a suitable amount such as at least 75 weight % or at least 80 weight %, and generally at no more than 90 weight %, based on the total weight of the aqueous inkjet ink composition.

Each aqueous inkjet ink composition described herein can be prepared by dispersing suitable polymer-dispersed pigments colorants in water and mixing in other noted materials such as the (b) compounds represented by Structure (I), (c) humectants or co-solvents, and any adjuvants, promoters, supplemental antimicrobial agents, and additional materials in suitable amounts.

Ink Sets

Ink sets can be provided by the present invention, and they can include two or more aqueous inkjet ink compositions, each of which comprises at least one visible polymer-dispersed pigment colorant (described above) to provide a desired color or hue. For example, each ink set can include useful aqueous inkjet ink compositions which can have different hues or “colors” such as various shades of orange, red, violet, green, cyan, yellow, black, magenta, brown, pink, and blue and thus contain one or more suitable polymer-dispersed pigment colorants suitable to provide the desired hue. Any desirable hue, for example as defined using known a* and b* CIELAB values, can be provided by proper use and formulation of suitable polymer-dispersed pigment colorants. Aqueous “white” inkjet ink compositions are also useful in certain situations and can be included in an ink set. A wide variety of organic and inorganic pigments can be used individually or in combination in such aqueous inkjet ink compositions, as described above, and they can have the desired pigment particle size as described above. The polymer-dispersed pigment colorants can be present in suitable amounts as described above for the aqueous inkjet ink compositions according to this invention

In addition to the polymer-dispersed pigment colorants, one or more of the aqueous color inkjet ink compositions in an ink set can include one or more aqueous-soluble dyes that are well known in the art, for example as described in Col. 12 (lines 4-55) of U.S. Pat. No. 8,455,570 (noted above).

The aqueous inkjet ink compositions in the ink sets can be formulated the same as or differently from those described above.

Each aqueous color inkjet ink compositions in an ink set can have a desirable pH of at least 7.5 and up to and including 11, or at least 8 and up to and including 10, as described above using suitable bases and buffer systems.

In addition, each aqueous color inkjet ink composition can have suitable dynamic viscosity of at least 1 centipoise (1 mPa-sec) but less than 5 centipoise (5 mPa-sec) as measured at 25° C.

Each of the aqueous inkjet ink compositions in an ink set independently comprises (has same or different): (a) a polymer-dispersed pigment colorant (as described above) in an amount of at least 0.9 weight % and up to and including 6 weight %, based on the total weight of the aqueous inkjet ink composition; a (b) composition consisting of compounds represented by Structure (I) as described above; and (c) at least one water-soluble humectant, co-solvent, or a combination of water-soluble humectant and co-solvent, all as described above and in the amounts described above.

For example, an ink set according to the present invention can have two or more of the following specific types of aqueous inkjet ink compositions:

(i) an aqueous inkjet ink composition comprising a polymer-dispersed cyan pigment colorant,

(ii) an aqueous inkjet ink composition comprising a polymer-dispersed magenta pigment colorant,

(iii) an aqueous inkjet ink composition comprising a polymer-dispersed yellow pigment colorant, and

(iv) an aqueous inkjet ink composition comprising a polymer-dispersed black pigment colorant.

In many ink sets, all four (i) through (iv) aqueous inkjet ink compositions are present.

In some embodiments, an ink set according to this invention can further comprise a particle-free colorless inkjet composition (or aqueous particle-free fluid), for example as described in U.S. Pat. No. 8,764,161 (noted above), the disclosure of which is incorporated herein by reference. Such compositions can be known as “fluids” in the art and can have various purposes or functions such as printhead maintenance, storage, flushing, or cleaning, or use as replenishment fluids. The details of such fluids are provided in the cited patent, the disclosure of which is incorporated herein by reference. By “particle-free,” it is meant that such compositions do not purposely contain particulates or pigments of any type, colorless or colored. Further details of useful such fluids are provided below in the discussion of individual aqueous particle-free “fluid sets.”

Such particle-free colorless inkjet compositions can also comprise a (b) composition consisting of compounds represented by Structure (I) described above, which compounds can be the same as or different from those used in the two or more aqueous inkjet ink compositions in an ink set containing a polymer-dispersed pigment colorant. For example, such particle-free colorless inkjet compositions can contain 2-phenoxyethanol, 2-phenylethanol, or both 2-phenoxyethanol and 2-phenylethanol [as when R is 2-phenyl or 2-phenoxy in Structure (I)] in an amount of at least 0.5 weight % and up to and including 2 weight %, based on the total weight of the particle-free colorless inkjet composition.

The durability, gloss, and other properties of an inkjet printed image can be improved by the application of a colorless polymeric overcoat composition, which can be considered an aqueous particle-free inkjet composition according to this invention. Examples of such compositions are provided in U.S. Pat. No. 7,219,989 (Uerz et al.). In other to achieve high inkjet printing speeds and throughput associated with CIJ printing, such an overcoat composition can be applied using a CIJ printhead following in-line with one or more printheads of drop-forming nozzles dispensing “colored” aqueous inkjet ink compositions. Further details about such application are provided in Col. 17 (lines 16-48) of U.S. Pat. No. 8,173,215 (noted above).

Additional aqueous particle-free inkjet compositions (or inks) that can be part of an ink set include those described in U.S. Patent Application Publication 2018/0051184 (Lussier et al.) that can be inkjet printed to provide colorless or colored coatings, the disclosure of which publication is incorporated herein by reference. Such compositions can comprise at the least, one or more anionic polyether polyurethanes or anionic acrylic or styrene-acrylic polymers as described above, as well as a suitable antifoamant or defoamer to reducing foaming propensities. Such aqueous particle-free inkjet compositions can further comprise a (b) composition having compounds defined by Structure (I) as described above.

Each component or composition present in an ink set, whether colored or colorless, may contain various other additives (such as defoamers, surfactants, conductivity enhancing agents, drying agents, water-fast agents, chelating agents, thickeners, anti-kogation agents, stabilizers, and buffering agents) that would be readily apparent to one skilled in the art.

Aqueous Particle-Free Fluids

Aqueous particle-free fluids can be designed and used in the practice of the present invention for various purposes. Each of these aqueous particle-free fluids generally independently has a dynamic viscosity of less than or equal to 5 centipoise (5 mPa-sec) at 25° C. for example as measured using a rolling ball viscometer, or a capillary viscometer, and standard procedures. Each of these aqueous particle-free fluids purposely contains no particles, colorless or otherwise, and can be used for various purposes or functions in inkjet printing methods, as described below.

Two or more of such aqueous particle-free fluids, having the same or different purposes, can be incorporated into a “fluid set” for commercialization if desired. Alternatively, one or more of the aqueous particle-free fluids can be included within the ink sets described above.

Each aqueous particle-free fluid independently can have a pH broadly similar to the aqueous inkjet ink compositions described above, that is, of at least 5 and up to and including 11. pH can be managed for each aqueous particle-free fluid using the teaching in Col. 11 (line 58) to Col. 12 (line 26) of U.S. Pat. No. 8,764,161 (noted above). In some embodiments, the pH is at least 10 and up to and including 11 and can be managed by the presence of one or more bases, as described below.

Each of the aqueous particle-free fluids according to the present invention independently consists essentially of a (b) composition consisting of one or more compounds represented by Structure (I) as defined above. Individual aqueous particle-free fluids in a fluid set can have the same or different (b) composition. The one or more compounds in the (b) composition, for example those wherein R is 2-phenyl or 2-phenoxy, can be present in an aqueous particle-free fluid in an amount of at least 0.5 weight % and up to and including 2 weight %, based on the total weight of the aqueous particle-free fluid.

In a particular embodiment, the aqueous particle-free fluids can have different general compositions aside from their (b) composition and are suitable for two or more different printing service functions. For example, a “replenisher” aqueous particle-free fluid can be used to restore evaporated solvent from the aqueous inkjet ink composition in the ink tank and return that composition to a previous colorant concentration. Such aqueous particle-free fluids generally consist essentially of only the (b) composition described above. However, it is possible for such aqueous particle-free fluids to additionally include a humectant, co-solvent, supplemental antimicrobial agent, surfactant, promoter, or any combination thereof, as described in more detail below. For example, such aqueous particle-free fluids can desirably also contain any volatile organic solvents that are lost to evaporation during ink recycling, or a weak organic base (for example, an alkanolamine) to mitigate pH drift due to carbonic acid formation resulting from carbon dioxide uptake. Since a continuous inkjet printer fluid system method of determining the relative concentration of the ink in the ink tank is to measure the ink's ionic conductivity (or alternatively its resistivity), it is undesirable to alter the restored ink fluid properties and ion inventory materially with contributions that accumulate from repeated replenisher addition. Thus, it is desired to minimize the replenisher aqueous particle-free fluid content of for example, organic solvents, humectants, pH and ionic conductivity modifying additives, polymer compounds, surfactants, and antifoamants.

A “maintenance” aqueous particle-free fluid, a printhead cleaner and storage fluid, can be used to re-dissolve, re-disperse, or solubilize dried ink deposits that form on and around the nozzle plate nozzles and interfere with inkjet straightness and inkjet stability. It can then be used to purge the printhead ink channels and external surfaces to flush ink away. The aqueous particle-free fluid is also suitable for keeping the wetted parts ready for efficient start-ups after periods of extended storage, but the various printhead and fluid system maintenance functions of flushing, cleaning, and storage can be accomplished by specialized individual aqueous particle-free fluids as desired.

In addition to a (b) composition, a printhead cleaner and storage aqueous particle-free fluid according to the present invention can desirably be comprised of an organic solvent that improves upon the effectiveness of the fluid vehicle (water) in penetrating and solvating dried pigment particles in deposits of dried aqueous inkjet ink composition. It may additionally contain optional additives that include but are not limited to a solubilizing agent, a co-solvent, viscosity modifiers, a base, an acid, a pH buffer, a chelating agent, a dispersant, a water-soluble or water-dispersible polymer, a corrosion inhibitor, a viscosity modifier, a penetrant, a wetting agent, an antifoamant, and a defoamer. Effective solvents for dried aqueous inkjet ink composition can be selected from the class of dynamic surface tension reducing co-solvents also known in the art as penetrants, where the dynamic surface tension reducing polar co-solvent agent is also considered a functional surface tension modifying agent (that is, it is aptly referred to as a “solvo-surfactant” that is a low molecular weight, volatile solvent with an amphiphilic composition capable of reducing fluid surface tension and capable of self-aggregating). On a molar and mass fraction basis, such solvo-solvents are less effective surface modifying agents than the traditional surfactants for reducing equilibrium surface tension. Solvo-surfactants employed for removal of dried aqueous inkjet ink composition desirably comprise an asymmetric polyhydric alcohol or mono-alkyl ether derived from a polyhydric alcohol. Specific examples of lower (C₁-C₄) mono-alkyl ethers and derivatives originating from polyhydric alcohols include but are not limited to ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, ethylene glycol monoethyl ether acetate, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, polyethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monopropyl ether and diethylene glycol monobutyl ether acetate, among others all as supplied as the DOWANOL, CELLUSOLVE® and CARBITOL® series of compounds by Dow Chemical Co. Such solvo-surfactants, alone or in combination, can be used at amounts of at least 0.1 weight % and up to and including 20 weight %, or even in an amount of at least 3 weight % and up to and including 6 weight %, all based on the total weight of the aqueous particle-free fluid.

Increased fluid pH can be beneficial to the removal of dried aqueous inkjet ink composition, and printhead cleaner aqueous particle-free pH is optionally raised with organic bases, such as aliphatic amines substituted with hydroxyl groups (for example, alkanolamines), including monoethanolamine, diethanolamine, 3-amino-1-propanol, N-methylethanolamine, N-benzyl-N-methylethanolamine, 2-(2-aminoethoxy)ethanol, N,N-dimethyl-2-(2-aminoethoxy)ethanol, N-methyldiethanolamine, N,N-dimethylethanolamine, and triethanolamine. High pH aqueous particle-free fluids can have a pH of at least 10 and up to and including 11.

The wetting of contaminated printhead parts is critical to successful cleaning. If sufficient solvo-surfactant is employed, no additional dynamic or static surface tension modifier may be needed. Otherwise, a surfactant can be included. Representative compounds of these types are described in Col. 12 (lines 27-62) of U.S. Pat. No. 8,764,161 (noted above), the disclosure of which is incorporated herein by reference. One or more surfactants that may be anionic, cationic, amphoteric or nonionic in nature can be present in the aqueous particle-free fluid in an amount of at least 0.01 weight % and up to and including 10 weight %, based on the total weight of the aqueous particle-free fluid. While any agent that serves to control the surface tension at large surface ages can be usefully employed, the surfactant desirably has a weight-normalized molecular weight below about 1,000 Daltons. While the charged surfactants may have slower diffusion in aqueous fluids due to the aggregated water of hydration which effectively increases their bound mass, the useful non-ionic surfactants desirably have a molecular weight above 350 Daltons, above 400 Daltons, or even above 500 Daltons, to ensure slow diffusion in bulk ink so as to allow distinct control of static or equilibrium surface tensions. Examples of suitable nonionic surfactants include but are not limited to, linear or secondary alcohol ethoxylates (such as the TERGITOL® 15-S and TERGITOL® TMN series of compounds available from Dow Chemical Company and the BRIJ® series of compounds from Croda International Plc.), ethoxylated alkyl phenols (such as the TRITON® series of compounds from Dow Chemical Company), fluoro surfactants (such as the ZONYL® compounds from DuPont; and the FLUORAD® compounds from 3M), fatty acid ethoxylates, fatty amide ethoxylates, ethoxylated and propoxylated block copolymers (such as the PLURONIC and TETRONIC® series of compounds from BASF Corp., ethoxylated and propoxylated silicone based surfactants (such as the SILWET® series of compounds from Momentive), alkyl polyglycosides (such as the GLUCOPONS compounds from BASF Corp.) and acetylenic diol polyethylene oxide surfactants (such as the SURFYNOL® family of compounds from Evonik Corp.). The polymeric surfactants may be water soluble or water dispersible depending in part on their tendency to aggregate. A useful surfactant blend suitable for dispersing pigments and re-dispersing dried aqueous inkjet ink compound is ZETASPERSE® 1600 (Evonik Corp.).

In addition, styrene-acrylic type polymers can be used in the printhead cleaner and storage fluid. The water-soluble or water-dispersible polymeric components can be the same or similar to the polymers described above for use as polymeric dispersants for pigment colorants, or the anionic polymers incorporated into aqueous inkjet ink compositions. Such materials are present in the aqueous inkjet ink compositions to improve physical durability of an inkjet printed image, or to improve other characteristics of the compositions such as colloidal stability to gear pump mediated recirculating filtration. For example, such water-soluble or water-dispersible polymeric components can be random or block copolymers having both hydrophilic and hydrophobic recurring units derived from the corresponding ethylenically unsaturated polymerizable monomers. They can be “acrylics” and “styrene-acrylics” that are derived primarily from styrene monomers, and (meth)acrylic acid, or (meth)acrylate ester monomers. Some useful water-soluble or water-dispersible polymeric components can be derived from the various monomers and teaching provided in Col. 7 (line 24) to Col. 8 (line 55) of U.S. Pat. No. 8,764,161 (noted above). Other useful (3) water-soluble or water-dispersible polymeric compounds can be water-soluble polymers having poly(ethylene-oxide) segments and water-dispersible polyurethanes, for example, as described in Col. 10 (lines 24-46) of U.S. Pat. No. 8,764,161 (noted above). Mixtures of two or more of the same or different classes of these materials can be used if desired.

One or more water-soluble or water-dispersible polymeric components can be present in each aqueous particle-free fluid independently in an amount of up to and including 20 weight %, or at least 0.2 weight % and up to and including 10 weight %, or even at least 0.5 weight % and up to and including 8 weight %, or more likely in an amount of at least 2 weight % and up to and including 5 weight %, all based on the total weight of the aqueous particle-free fluid.

The aqueous particle-free fluids used for printhead cleaning and storage service desirably contain a humectant, co-solvent, or both, to facilitate inkjet printing start-up after an extended period of time without printing. Any water-soluble humectant or co-solvent known in the inkjet art and compatible with the other requirements of the invention can be employed. By water-soluble is meant that a mixture of the employed humectant(s) or co-solvent(s) and water are adequately homogeneous and not subject to spontaneous phase separation. While an individual humectant or co-solvent can be employed, useful aqueous particle-free fluid compositions can employ mixtures of two, three, or more humectants and co-solvents, each of which imparts a useful property. Representative examples of humectants and co-solvents used in aqueous-based ink compositions include (1) alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and tetrahydrofurfuryl alcohol; (2) polyhydric alcohols, such as ethylene glycol, di(ethylene glycol), tri(ethylene glycol), tetra(ethylene glycol), propylene glycol, di(propylene glycol), the poly(ethylene glycol)s with average molecular weights ranging from 200 to about 5000 Daltons (particularly poly(ethylene glycol)-400 (average M_(n) of about 400, herein referred to as PEG-400 for convenience)), the polypropylene glycols with average molecular weights ranging from 200 to about 5000 Daltons (particularly poly(propylene glycol)-425 (average M_(n) of about 425)), 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2,4-butanetriol, 3-methyl-1,3-butanediol, 2-methyl-1,3-propanediol, 1,5-pentanediol, 1,6-hexanediol, 2-methyl-2,4-pentanediol, 1,7-heptanediol, 2-ethyl-1,3-hexanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,8-octanediol, glycerol, 1,2,6-hexanetriol, 2-ethyl-2-hydroxymethyl-propanediol, 2-methyl-2-hydroxymethyl-propanediol, saccharides and sugar alcohols and thioglycol; (3) polyoxygenated polyols and their derivatives such as diglycerol, polyglycerols, glycerol ethoxides, glycerol propoxides, glyceryths, alkylated and acetylated glyceryths, saccharide such as sorbitol or fructose, pentaerythritol, pentaerythritol ethoxides, and pentaerythritol propoxides and their alkylated and acetylated derivatives. Particularly desirable ingredients serving primarily as a humectant to retard aqueous particle-free fluid drying and aid aqueous inkjet ink composition redispersability include glycerol, ethylene glycol, diethylene glycol, triethylene glycol, related polyols, and the polyhydric alcohol derivatives thereof, which are desirable. Triethylene glycol is particularly useful. The total humectant and co-solvent level of the fluid or ink is the sum of the individual contributions of humectant or miscible polar organic co-solvent, DST-modifying co-solvent (solvo-surfactant), and any other co-solvent ingredients, which may include humectant or organic co-solvent added directly or incidentally during the totality of aqueous particle-free fluid or ink formulation (for example, co-solvent associated with a commercial biocide preparation as a supplemental ingredient, or with a commercial pigment dispersion preparation that may be present to prevent so-called “paint-flakes” of dried pigment cake forming around a bottle cap, as described in U.S. Publication No. 2005/0075415 (Harz et al). The one or more (c) humectants, co-solvents, or both humectants and co-solvents can be present in the aqueous particle-free fluid in an amount of less than 20 weight %, or of at least 0.5 weight %, or at least 1 weight % and up to and including 15 weight %, or at least 3 weight % and up to and including 10 weight %, all based on the total weight of the aqueous particle-free fluid. More desirably, the total humectant and co-solvent level is less than or equal to 10 weight %.

Each aqueous particle-free fluid can further contain one or more promoters for the one or more compounds in the (b) composition potentially to enhance the effectiveness of the compounds represented by Structure (I). As described above, such promoters are generally alkane diols, each having at least 7 carbon atoms and up to and including 12 carbon atoms, and particularly having at least 7 carbon atoms and up to and including 10 carbon atoms. Representative useful compounds that can be used singly or in combination as promoters are described above. 1,2-Octanediol is particularly useful as a promoter in this regard.

One or more promoters can be present in each aqueous particle-free fluid at a total amount of less than or equal to 1.5 weight % or less than or equal to 1.25 weight %, based on the total weight of the aqueous inkjet ink composition. A minimum amount can be at least 0.3 weight %.

One or more (d) supplemental antimicrobial compounds can also be present independently in each aqueous particle-free fluid in a fluid set, and such materials are different from the (b) composition described above. Representative materials include but are not limited to, iodopropynyl butyl carbamate (CAS 55406-53-6), piroctone olamine (CAS 68890-66-4), 2,4-dichlorobenzyl alcohol (CAS 1777-82-8), boric acid (CAS 10043-35-3) and monovalent and divalent metal ion salts derived from boric acid, and combinations of any of these materials. A useful amount of the one or more supplemental antimicrobial agents is at least 0.01 weight % and up to and including 3 weight %, based on the total weight of the aqueous particle-free fluid.

Methods of Inkjet Printing

Details regarding the various methods and apparatus useful for inkjet printing are provided in numerous publications, but none of that art describes such methods being carried out using the (b) composition having compounds represented by Structure (I) defined above, or the advantages achieved according to the present invention.

Methods according to the present invention are carried out firstly by provide a suitable substrate (also known in the art as “receiver” or “receiver element”). Any individual substrate will have what would be understood as having at least one surface with a “printable” area onto which an aqueous inkjet ink composition can be ink jetted using suitable equipment and processes.

Suitable substrates are typically planar in nature with two opposing surfaces or supporting sides, one or both of which can be inkjet-printed to provide the same or different images. Substrates can have a single “layer” or stratum or be composed of multiple layers or strata composed of the same or different materials. In most instances, a substrate has a predominant material, such as a cellulosic material that is coated or layered with one or more other types of materials such as polymeric coatings.

For example, representative substrate can include but are not limited to, photoglossy receivers or plain paper receivers such as bright white inkjet papers that are commercially available from various commercial sources (as described for example, in Col. 13, lines 28-34) of U.S. Pat. No. 8,187,371 (noted above). The photoglossy receivers (also known as swellable media or microporous media) can be manufactured with a coated layer on an underlying paper support and are also useful for providing photographic quality inkjet-printed images. Some details of such substrates are provided in Col. 13 (lines 37-51) of U.S. Pat. No. 8,187,371 (noted above). Plain papers can be treated with multivalent salts during or after manufacture as is well known in the art. Other useful substrates are described in U.S. Pat. No. 6,045,917 (Missell et al.), U.S. Pat. No. 5,605,750 (Romano et al.), U.S. Pat. No. 5,723,211 (Romano et al.), and U.S. Pat. No. 5,789,070 (Shaw-Klein et al.), the disclosures of all of which are incorporated herein by reference.

Useful paper substrates include plain bond papers, surface-treated papers, coated or calendared business gloss papers, resin-coated papers, laminated substrates comprising both paper layers and polymeric film layers such as polyester film layers, and heavy stock papers. It is also possible to use fabrics, cardboard or paperboard materials, organic or inorganic polymeric materials, microporous materials, non-woven synthetics, and any other substrate material known in the art to form inkjet printable substrates. A substrate can be transparent, translucent, or opaque, and it can be provided for inkjet printing in the form of a rigid or semi-rigid sheet, cut or continuous film or web, or wound roll.

Durability and other properties of inkjet-printed color images can be improved by using substrates that have been pretreated with a composition to enhance the quality of the resulting images. This pretreatment is typically done prior to incorporation of the substrate into the inkjet printing apparatus (such as a continuous inkjet printing apparatus), but in some instances, the substrate can be pretreated within the apparatus before inkjet printing with one or more aqueous inkjet ink compositions. One or both opposing surfaces (planar sides) of a substrate can be pretreated, or one supporting surface can be pretreated and the opposite supporting surface left untreated.

For example, a substrate can be pretreated with a pretreatment composition comprising a water-soluble multivalent metal ion salt, such as but not limited to, a salt comprising one or more multivalent cations including calcium, magnesium, barium, zinc, and aluminum cations, with calcium and magnesium cations being particularly useful. Examples of useful multivalent metal cation salts to provide such cations are known in the art as useful salts can be determined by a skilled artisan. Details of such pretreatment procedures and compositions are provided for example, in U.S. Pat. No. 9,067,448 (Dannhauser et al.), the disclosure of which is incorporated herein by reference.

Each aqueous inkjet ink composition according to the present invention [as described above having the required viscosity and all (a) polymer-dispersed pigment colorant(s), (b) composition consisting of Structure (I) compounds, and (c) compounds] can be inkjet-printed from a suitable printhead in a controlled manner onto at least one surface of the substrate to provide an inkjet-printed image on that surface of the substrate.

While the aqueous inkjet ink compositions according to this invention may be useful in one or more DOD printing systems, the advantages are particularly evident when the present invention is carried out using CIJ printing processes and equipment. There are several CIJ printing processes known in the art, and the present invention is not limited to a CIJ process, but there may be certain CIJ processes that are more useful than others in carrying out the present invention. In general, such CIJ processes use one or more aqueous inkjet ink compositions that are ejected through one or more printheads (containing nozzles) and unprinted aqueous inkjet ink composition is collected and recycled through the printing system multiple times until it is used up. In addition, the CIJ printing system can have incorporated replenisher systems. Details of such CIJ processes and equipment are provided for example in U.S. Pat. No. 8,173,215 (noted above).

Thus, in most CIJ inkjet printing processes, each aqueous inkjet ink composition according to the present invention is ejected or printed from a main fluid supply dedicated to it only, as a continuous stream of the aqueous inkjet ink composition that is broken into both printing drops and non-printing drops. The non-printing drops of each aqueous inkjet ink composition can be collected using suitable collecting means such as a “catcher” and returned to its respective main fluid supply. This entire scenario can be carried out using a single (first) aqueous inkjet ink composition alone, or in combination with one or more “additional” aqueous inkjet ink compositions having the same or different “colors” or hues as the first aqueous inkjet ink composition. The multiple aqueous inkjet ink compositions are then inkjet printed in a chosen sequence that can be controlled by particular software and digital input, in a controlled manner, to provide a multicolor inkjet-printed image on the surface of the substrate.

In addition, inkjet printing of an aqueous “colorless” or aqueous particle-free composition or fluid (as described above) can be carried out simultaneously or sequentially with inkjet printing of the “colored” aqueous inkjet ink composition(s). For example, according to U.S. Patent Application Publication 2018/0051184 (Lussier et al.), a colorless lacquer or protective coating can be applied over single- or multi-color inkjet-printed images.

As is implied herein, each aqueous inkjet ink composition, aqueous colorless composition, or aqueous particle-free composition of fluid used in such processes can be designed according to the present invention to comprise the described (a) one or more polymer-dispersed pigment colorants (only for the aqueous inkjet ink compositions), described (b) composition consisting of compounds defined by Structure (I), and described (c) compounds. However, all such aqueous compositions used in the method according to the present invention can further include one or more (d) supplemental antimicrobial compounds that are all different from the (b) composition.

Useful CIJ printing processes and equipment can include replenishment systems that measure ink electrical resistivity and are described for example in U.S. Pat. No. 5,526,026 (Bowers), the disclosure of which is incorporated herein by reference and in EP 0597628B1 (Loyd et al.). Useful CIJ printing processes and equipment that employ other means for ink concentration sensing are disclosed in U.S. Pat. No. 7,221,440 (McCann et al.), the disclosure of which is incorporated herein by reference, and in EP 0571784B1 (McCann et al.) and EP 1,013,450B1 (Woolard et al.).

In one embodiment, basic replenishment is carried out as follows: a fluid system contains an ink resistivity measurement cell through which aqueous inkjet ink composition passes as it is being recirculated through the ink handling portion of the system, including the printhead. A calculation means determines the resistance of the ink resistivity cell. A logic and control unit, responsive to the calculation means, controls the transfer of aqueous inkjet ink composition from a supplemental “ink” supply and the transfer of an aqueous particle-free fluid (“carrier fluid”) from a replenishment carrier fluid supply to the system main fluid supply, to maintain desired resistivity in the aqueous inkjet ink composition. The volume of the aqueous inkjet ink composition is monitored by a float valve position, and when a predetermined volume has been depleted, the predetermined volume is replaced by either aqueous inkjet ink composition from the supplemental “ink” supply or by carrier fluid from the replenishment carrier fluid supply.

Thus, the first and any additional aqueous inkjet ink compositions can be replenished, respectively, with first and any additional aqueous inkjet ink compositions, each of which consists essentially of the same or different (a), (b), and (c) components described above, and especially the same or different (b) composition that consists of compounds defined by Structure (I) noted above.

In other examples, the method according to the present invention can further comprise replenishing a main fluid supply with an aqueous particle-free fluid that has a dynamic viscosity of less than or equal to 5 centipoise (5 mPa-sec) at 25° C., which aqueous particle-free fluid consists essentially of:

the same or different (b) composition that is present in the first aqueous inkjet ink composition, as described above.

Such aqueous particle-free fluid can further comprise a (d) supplemental antimicrobial compound, as described above, that is different from the (b) composition. For example, the (d) supplemental antimicrobial compound can be iodopropynyl butyl carbamate, piroctone olamine, 2,4-dichlorobenzyl alcohol, boric acid, or a combination of these compounds.

The aqueous particle-free fluid used for replenishment can also contain one or more promoters for the one or more (b) compounds, each of which promoter is an alkane diol having a least 7 carbon atoms and up to and including 12 carbon atoms, in an amount of less than or equal to 1.5 weight %, based on the total weight of the aqueous inkjet ink composition. For example, the promoter can be 1,2-octanediol.

In some embodiments, the method according to the present invention is carried out using a plurality of printing drops formed from a continuous fluid stream, and non-printing drops of a different volume than the printing drops are diverted by a drop deflection means into a “gutter” for collection and recirculation. Details about such CIJ printing systems and equipment are provided for example in U.S. Pat. No. 6,588,888 (Jeanmaire et al.), U.S. Pat. No. 6,554,410 (Jeanmaire et al.), U.S. Pat. No. 6,682,182 (Jeanmaire et al.), U.S. Pat. No. 6,793,328 (Jeanmaire et al.), U.S. Pat. No. 6,866,370 (Jeanmaire et al.), U.S. Pat. No. 6,575,566 (Jeanmaire et al.), and U.S. Pat. No. 6,517,197 (Hawkins et al.), and in U.S. Patent Application Publications 2002/0202054 (Jeanmaire et al.), the disclosures of all of which are incorporated herein by reference.

In other embodiments, an aqueous inkjet ink composition can be printed using an apparatus capable of controlling the direction of the formed printing drops and non-printing drops by asymmetric application of heat to the fluid stream that initializes drop breakup and serves to steer the resultant drop as described for example in U.S. Pat. No. 6,079,821 (Chwalek et al.) and U.S. Pat. No. 6,505,921 (Chwalek), the disclosure of both of which are incorporated herein by reference. Useful agitation, heated supply, printhead, and fluid filtration means for CIJ printing are described for example in U.S. Pat. No. 6,817,705 (Crockett et al.), the disclosure of which is incorporated herein by reference.

A simple schematic of a CIJ printing system is provided in FIG. 1 of U.S. Pat. No. 8,764,161 (noted above).

Moreover, in some embodiments, the method according to the present invention further comprises:

stopping the inkjet printing of the first aqueous inkjet ink composition;

delivering an aqueous particle-free fluid (as described above) from a maintenance fluid supply to the printhead; and

ejecting the aqueous particle-free fluid from the printhead to purge the first aqueous inkjet ink composition from the printhead,

-   -   wherein the aqueous particle-free fluid that has a dynamic         viscosity of less than or equal to 5 centipoise (5 mPa-sec) at         25° C. as measured using a rolling ball viscometer and consists         essentially of:     -   the same or different (b) composition that is present in the         first aqueous inkjet ink composition, as described above.

Such aqueous particle-free fluid can further contain less than or equal to 15 weight % of one or more compounds selected from water-soluble humectants, co-solvents, and both water-soluble humectants and co-solvents, based on the total weight of the aqueous particle-free fluid.

Moreover, it can further contain one or more promoters for the (b) composition, each of which promoter is an alkane diol having a least 7 carbon atoms and up to and including 12 carbon atoms, in an amount of less than or equal to 1.5 weight %, based on the total weight of the aqueous particle-free fluid.

The aqueous particle-free fluid used in this series of steps can be considered a “maintenance fluid” that is used to clean the printheads at selected intervals or when a printing job is completed or interrupted for some reason. The maintenance fluid can be stored in a printhead for a period of time, and delivery of first or additional aqueous inkjet ink compositions from the main and any other respective fluid supplies to the respective printheads can be subsequently restarted for a new printing procedure.

Further details about use of an aqueous particle-free composition as a maintenance fluid in this manner are provided in U.S. Pat. No. 8,764,161 (noted above).

According to some embodiments of this invention, a method of printing an image using a continuous inkjet printer system, can comprise:

providing a substrate (as described above);

providing a jetting module having a plurality of nozzles including a nozzle in fluid communication with a main fluid supply containing a first aqueous inkjet ink composition [such equipment would be readily known to one skilled in the art in view of teaching in U.S. Pat. No. 9,010,909 (Nelson et al.) the disclosure of which is incorporated herein by reference];

causing drops of the first aqueous inkjet ink composition to be formed while it is jetted through the nozzle in fluid communication with the main fluid supply by drop stimulation in response to time-varying electrical signals [equipment used to perform this function would be readily apparent to one skilled in the art in view of teaching in U.S. Pat. No. 9,010,909 (Nelson et al.)];

providing a catcher including a drop contact face [known to one skilled in the art in view of teaching in U.S. Pat. No. 9,010,909 (noted above)];

using a deflection mechanism to deflect at least some of the drops of the first aqueous inkjet ink composition onto the drop contact face of the catcher while allowing other drops of the first aqueous inkjet ink composition to pass by the catcher and to be deposited onto a surface of the substrate [known to one skilled in the art in view of teaching in U.S. Pat. No. 9,010,909 (noted above)]; and

causing the drops of the aqueous inkjet ink composition that contact the drop contact face to flow along the drop contact face [known to known skilled in the art in view of teaching in U.S. Pat. No. 9,010,909 (noted above)],

-   -   wherein the first aqueous inkjet ink composition has a dynamic         viscosity of less than or equal to 5 centipoise (5 mPa-sec) at         25° C., and consists essentially of:     -   (a) one or more polymer-dispersed pigment colorants (as         described above) in a total amount of at least 0.9 weight % and         up to and including 6 weight %, based on the total weight of the         aqueous inkjet ink composition;     -   (b) a composition (as described above) consisting of one or more         compounds represented by the following Structure (I):

HO—CH₂—CH₂—R  (I)

-   -   wherein R is a substituted or unsubstituted phenyl group or a         substituted or unsubstituted phenoxy group, in a total amount of         at least 0.5 weight % and up to and including 2 weight %, based         on the total weight of the aqueous inkjet ink composition;     -   (c) less than or equal to 15 weight % of one or more compounds         (as described above) selected from water-soluble humectants,         co-solvents, and both water-soluble humectants and co-solvents,         based on the total weight of the aqueous inkjet composition,

wherein each of the one or more polymer-dispersed pigment colorants has a 50^(th) percentile particle diameter of less than 70 nm and a 95^(th) percentile particle diameter of less than 150 nm, all particle diameters being measured using a dynamic light scattering particle size analyzer.

In some embodiments, the least some of the drops (deflected drops) of the first aqueous inkjet ink composition used in the noted method, are smaller than the other drops (non-deflected and deposited drops) of the first aqueous inkjet ink composition.

In still other embodiments, the least some of the drops (deflected drops) of the first aqueous inkjet ink composition are larger than the other drops (non-deflected and deposited drops) of the first aqueous inkjet ink composition.

Moreover, a method according to the present invention for continuous inkjet (CIJ) printing can comprise:

supplying a main fluid supply of a continuous inkjet printer (as known in the art) with a first aqueous inkjet ink composition (as described above) having a dynamic viscosity of less than or equal to 5 centipoise (5 mPa-sec) at 25° C., and consisting essentially of:

-   -   (a) one or more polymer-dispersed pigment colorants (as         described above) in a total amount of at least 0.9 weight % and         up to and including 6 weight %, based on the total weight of the         aqueous inkjet ink composition;     -   (b) a composition (as described above) consisting of one or more         compounds represented by the following Structure (I):

HO—CH₂—CH₂—R  (I)

-   -   wherein R is a substituted or unsubstituted phenyl group or a         substituted or unsubstituted phenoxy group, in a total amount of         at least 0.5 weight % and up to and including 2 weight %, based         on the total weight of the aqueous inkjet ink composition;     -   (c) less than or equal to 15 weight % of one or more compounds         (as described above) selected from water-soluble humectants,         co-solvents, and both water-soluble humectants and co-solvents,         based on the total weight of the aqueous inkjet composition,     -   wherein each of the one or more polymer-dispersed pigment         colorants has a 50^(th) percentile particle diameter of less         than 70 nm and a 95^(th) percentile particle diameter of less         than 150 nm, all particle diameters being measured using a         dynamic light scattering particle size analyzer;

ejecting a continuous stream of drops of the first aqueous inkjet ink composition from a drop generator mechanism (as known in the art);

in response to electrical signals received from a control mechanism, selecting between printing drops for imaging a substrate and nonprinting drops that are collected and returned to the main fluid supply, both types of drops from the first aqueous inkjet ink composition; and

replenishing the main fluid supply as a function of resistivity of the first aqueous inkjet ink composition in the main fluid supply. Replenishment can be achieved using replenishing first aqueous inkjet ink composition, or a replenishing aqueous particle-free fluid, as described above.

Additional CIJ printing processes and details of useful CIJ apparatus are described for example in U.S. Pat. No. 8,585,189 (Marcus et al.), U.S. Pat. No. 8,651,632 (Marcus et al.), U.S. Pat. No. 8,696,094 (Marcus et al.), U.S. Pat. No. 8,888,256 (Marcus), and U.S. Pat. No. 9,969,178 (Roberts et al.), all of which disclosures are incorporated herein by reference.

Printed Articles

A resulting inkjet-printed article (or printed receiver element) can have a monochrome or multi-color image on at least one supporting surface of an article, including but not limited to, documents, paper currency, postage stamps, packaging materials, fabrics, polymeric films or sheets, non-woven webs or sheets, labels for clothing, perfume, or wine bottles labels, lottery tickets, passports, drivers licenses, and other articles or papers that would be readily apparent to one skilled in the art using the teaching provided herein.

The present invention provides at least the following embodiments and combinations thereof, but other combinations of features are considered to be within the present invention as a skilled artisan would appreciate from the teaching of this disclosure:

1. A fluid set comprising two or more aqueous particle-free fluids, each of which independently has a dynamic viscosity of less than or equal to 5 centipoise (5 mPa-sec) at 25° C., and each of which two or more aqueous particle-free fluids independently consists essentially of:

(b) a composition consisting of one or more compounds represented by the following Structure (I):

HO—CH₂—CH₂—R  (I)

wherein R is a substituted or unsubstituted phenyl group or a substituted or unsubstituted phenoxy group, in a total amount of at least 0.5 weight % and up to and including 2 weight %, based on the total weight of the aqueous inkjet ink composition.

2. The fluid set of embodiment 1, wherein each of the two or more aqueous particle-free fluids further contains one or more promoters for the one or more Structure (I) compounds of the (b) composition, each of which promoters is an alkane diol having a least 7 carbon atoms and up to and including 12 carbon atoms, in an amount of less than or equal to 1.5 weight %, based on the total weight of the aqueous inkjet ink composition.

3. The fluid set of embodiment 2 wherein the promoter is 1,2-octanediol.

4. The fluid set of any of embodiments 1 to 3, wherein R is either an unsubstituted phenyl group or an unsubstituted phenoxy group.

5. The fluid set of any of embodiments 1 to 4, wherein at least one of the two or more aqueous particle-free fluids further comprises at least one water-soluble or water-dispersible polymeric component in an amount of up to and including 20 weight %, based on the total weight of the aqueous particle-free fluid, which is an acrylic polymer, a styrene-acrylic polymer, water-soluble polymer having poly(ethylene oxide) segments, water-dispersible polyurethane, or two or more of such materials.

6. The fluid set of any of embodiments 1 to 5, wherein at least one of the two or more aqueous particle-free fluids has a pH of at least 8 and up to and including 11.

7. The fluid set of any of embodiments 1 to 6, wherein at least one of the two or more aqueous particle-free fluids comprises an anionic, nonionic, cationic, or amphoteric surfactant in an amount of least 0.01 weight % and up to and including 5 weight %, based on the total weight of the aqueous particle-free fluid.

8. The fluid set of any of embodiments 1 to 7, wherein at least one of the two or more aqueous particle-free fluids comprises a (d) supplemental antimicrobial agent that is different from the (b) composition.

9. The fluid set of embodiment 8, wherein the (d) supplemental antimicrobial agent is iodopropynyl butyl carbamate, piroctone olamine, 2,4-dichlorobenzyl alcohol, boric acid or a metal ion salt derived from boric acid, or a combination of these compounds.

10. An aqueous particle-free fluid having a dynamic viscosity of less than or equal to 5 centipoise (5 mPa-sec) at 25° C., and comprising:

(b) a composition consisting of one or more compounds represented by the following Structure (I):

HO—CH₂—CH₂—R  (I)

wherein R is a substituted or unsubstituted phenyl group or a substituted or unsubstituted phenoxy group, in a total amount of at least 0.5 weight % and up to and including 2 weight %; and

a solvo-surfactant in an amount of at least 0.1 weight % and up to and including 20 weight %,

all amounts being based on the total weight of the aqueous particle-free fluid.

11. The aqueous particle-free fluid of embodiment 10, wherein the solvo-surfactant is an asymmetric polyhydric alcohol or mono-alkyl ether derived from a polyhydric alcohol.

12. The aqueous particle-free fluid of embodiment 10 or 11, wherein the solvo-surfactant is a C₁ to C₄ mono-alkyl ethyl derived from a polyhydric alcohol.

13. The aqueous particle-free fluid of any of embodiments 10 to 12, wherein the solvo-surfactant is present in an amount of at least 3 weight % and up to and including 6 weight %, based on the total weight of the aqueous particle-free fluid.

14. The aqueous particle-free fluid of any of embodiments 10 to 13, which has a pH of at least 8 and up to and including 11, and further comprises an aliphatic amine.

15. The aqueous particle-free fluid of any of embodiments 10 to 14, further comprising a surfactant different from the solvo-surfactant, which surfactant is present in an amount of at least 0.01 weight % and up to 10 weight %, based on the total weight of the aqueous particle-free fluid.

16. The aqueous particle-free fluid of any of embodiments 10 to 15, further comprising a humectant or co-solvent that is different from the solvo-surfactant.

The following working examples are provided to illustrate the practice of one or more aspects and embodiments of the present invention.

In the following preparations and examples, the pigment dispersion reported pigment content is based on the weight percent of the as-received colorant in the final dispersion.

Preparation of Aqueous Inkjet Ink and Service Fluid Compositions Polymeric Dispersant and Additive Preparation:

Polymeric Dispersant P-1

In a representative procedure, a 5-liter, three-necked, round bottom flask equipped with a mechanical stirrer, a reflux condenser, and a gas inlet was charged with 225 g of 1-methoxy-2-propanol and was sparged with nitrogen. Akzo-Nobel Chemicals, Inc., initiator PERKADOX® AMBN-GR (1.9 g) was added with stirring. A reactant reservoir was charged with 225 g of 1-methoxy-2-propanol, 23.4 g of 1-dodecanethiol, 203.5 g of benzyl methacrylate, 165.0 g of stearyl methacrylate, and 181.5 g of methacrylic acid, and the solution was degassed by nitrogen sparging. PERKADOX® AMBN-GR (7.7 g) was added and mixed in. The reactor temperature was raised to 77° C. and the reactants were pumped from the reservoir at a about 2.3 ml/min over a 360-minute period. The reaction mixture was then stirred for at least 12 hours at about 77° C. The resulting polymer was neutralized to completion with N,N-dimethylethanolamine and stirred for 45 minutes. The reaction mixture was diluted with 2,580 g of water and filtered through a Pall Corp. UILTIPLEAT® polypropylene cartridge filter. The final polymer solution of Polymeric Dispersant P-1 had a concentration of about 20 weight % solids and its pH was 8.6. The polymer weight-average molecular weight was 9,070 Daltons.

Polymeric Dispersant P-2

Polymeric dispersant P-2 was prepared in a similar fashion to P-1, except that 90% of the acid was reacted with potassium hydroxide during the neutralization step instead of with one equivalent of N,N-dimethylethanolamine. The final polymer solution of Polymeric Dispersant P-2 had a concentration of about 17 weight % solids.

Polymeric Additive P-3

A benzyl methacrylate-methacrylic acid copolymer having monomer weight ratio 77:23 and acid number of about 137 was 90%-neutralized with potassium hydroxide to provide an aqueous solution. The final polymer solution of Polymeric Additive P-3 had a concentration of about 25 weight % solids.

Polymeric Additive P-4

BASF Dispersions & Pigments North America JONCRYL® HPD 696, which is a styrene acrylic copolymer having a weight-average molecular weight M_(w) of 16,000 Daltons, was 90%-neutralized with potassium hydroxide to provide an aqueous solution. The final polymer solution of Polymeric Additive P-4 had a concentration of about 20 weight % solids.

Polymeric Additive P-5

In a 50-liter, round bottom flask equipped with thermometer, stirrer, water condenser, nitrogen inlet, and a vacuum outlet were placed 1,454.4 g of TERATHANE® 2000 polyether glycol, 670.5 g of 2,2-bis(hydroxymethyl) propionic acid, 313.2 g of 1,4-butanediol, and 3,771 g of ethyl acetate. The temperature was adjusted to 65° C., and when a homogeneous solution was obtained, 1,840.9 g of isophorone diisocyanate was added, followed by 184 g of ethyl acetate. The temperature was raised to 78° C. and maintained for 22 hours to complete the reaction. The reaction mixture was then diluted with 86 g of 2-propanol before being neutralized with 467.9 g of N,N-dimethylethanolamine. Under high shear, 18 kg of distilled water was added and the organic solvents were subsequently removed by distillation under vacuum. The resultant aqueous dispersion was filtered and was determined to have a non-volatile solids concentration of about 25 weight % and a pH value of about 8.0. The weight-average molecular weight (M_(w)) of the polyurethane dispersion was found by size exclusion chromatography to be about 19,800.

Pigment Dispersion Preparations: Pigment Dispersion KD-1

To a 2.5-gallon (9.46 liter), 9-inch (22.9 cm) diameter and 12-inch (30.5 cm) deep, double-walled stainless-steel mixing vessel containing four baffles were added water (1,000 g) and a solution of Polymeric Dispersant P-1 (1,000 g of a 19.9 weight % solution). A nominal 4-inch (10.2 cm), ring-style disperser impeller (Hockmeyer Equipment Corp. D-Blade) driven by a Charles Ross & Son Co. Model HSM-100LH-2 High Shear Mixer was centered 2 inches (5.1 cm) above the bottom of the mixing vessel and stirring was initiated. Cabot Corp. BLACK PEARLS® 900 carbon black pigment (500 g) was slowly integrated into the fluid. Milling media comprising beads of polystyrene resin (copolymer derived from styrene and a divinyl benzene/ethylvinyl benzene mixture) with an average particle diameter of 50 μm (3,000 g) was added slowly while increasing impeller speed. The mixture was milled with an impeller blade tip speed of about 19 m/sec for about 20 hours at an internal temperature of 25-35° C. Samples were periodically removed, diluted, and filtered for particle size determination by a Microtrac, Inc., NANOTRAC® NPA 150 dynamic light scattering particle size analyzer. When milling was complete, the dispersion/media milling mixture was further diluted with a solution of water (1,667 g) to a final pigment concentration of about 12 weight %, a polymeric dispersant concentration of about 4.8 weight % including the counterion, and a theoretical dispersion batch size of about 4,167 g. The impeller was removed, and the milling media was separated from the dispersion by filtration. A final filtration through a 0.3-μm removal efficiency Pall Corp. PROFILE II® depth filter gave roughly 4 kg of dispersion, approximately 80% yield. The volume-weighted 50^(th) percentile particle size distribution diameter was about 55 nm, and the 95^(th) percentile particle size distribution diameter was about 99 nm as characterized by the NANOTRAC® NPA 150 dynamic light scattering particle sizing instrument.

Pigment Dispersion MD-1

Magenta pigment dispersion MD-1 was prepared in a similar manner to pigment dispersion KD-1, except that BASF Dispersions & Pigments North America CINQUASIA® Magenta D 4500 J was used in place of carbon black pigment. The resulting dispersion had approximately 12 weight % pigment and 6.1 weight % polymeric dispersant, including the counterion. The volume-weighted median particle size was about 16 nm and the 95^(th) percentile particle size distribution diameter was about 59 nm as characterized by the NANOTRAC® NPA 150 dynamic light scattering particle sizing instrument.

Pigment Dispersion CD-1

Cyan pigment dispersion CD-1 was prepared in a similar manner to pigment dispersion KD-1, except that Pigment Blue 15:4 and Pigment Green 7 were used in a ratio of 3.75:1 in place of carbon black pigment and polymeric dispersant P-2 was used in place of P-1 in the presence of Lubrizol Corp. SOLSPERSE® 12000 and Polymeric Additive P-3. The resulting dispersion had approximately 12 weight % pigment and 8.5 weight % polymer dispersant, including the counterion. The volume-weighted 50^(th) percentile particle size distribution diameter is about 28 nm, and the 95^(th) percentile particle size distribution diameter is about 86 nm as characterized by the NANOTRAC® NPA 150 dynamic light scattering particle sizing instrument.

Pigment Dispersion YD-1

To a 10-gallon (37.85 liter), 13-inch (33 cm) diameter and 17-inch (43.2 cm) deep, double-walled stainless-steel mixing vessel containing four baffles were added 2,560 g water and 2,400 g of a 15% solution of Polymeric Dispersant P-2. A nominal 6-inch (15.2 cm) ring-style disperser impeller (Hockmeyer Equipment Corp. D-Blade) driven by a Hockmeyer Model HBI-7.5-11-99 High Shear Mixer was centered 3 inches (7.6 cm) above the bottom of the mixing vessel and stirring was initiated. Sun Chemical Co. Pigment Yellow 74 (1,200 g) was slowly added to the fluid. Milling media comprising polymeric beads derived from styrene and a divinyl benzene/ethylvinyl benzene mixture with an average particle diameter of 50 μm (7,200 g) were added slowly while increasing impeller speed. The mixture was milled with an impeller blade tip speed of about 20 meters/second for about 20 hours at an internal temperature of 25-30° C. The dispersion/media mixture was further diluted with water (6,000 g) to a final pigment concentration of about 12 weight % and a Polymeric Additive P-2 concentration of about 4.1 weight %. The impeller was removed, and the dispersion was separated from the milling media by filtration. A final filtration through a 0.3 μm particle removal rating Pall Corp. PROFILE II® depth filter yielded roughly 10.6 kg of dispersion. The volume-weighted 50^(th) percentile particle size distribution diameter of the dispersion was about 11 nm, and the 95^(th) percentile particle size distribution diameter was about 16 nm as determined by a NANOTRAC® NPA 150 dynamic light scattering particle sizing instrument.

Preparation of Continuous Inkjet Ink and Service Fluid Compositions: Aqueous Black Inkjet Ink Composition

Black pigmented CIJ aqueous inkjet ink composition K-A (E) was prepared using pigment dispersion KD-1 by combining the components at the relative proportions reported in the following TABLE I. In a representative procedure, 15.0 kg of aqueous inkjet ink composition was mixed by adding the components individually to a 30-liter cross-linked, high density polyethylene flat bottom tank using a 2-inch (5.1 cm) impeller rotating at about 1,000 rpm to provide good mixing. The ingredients (if so indicated) were added in the following functional component order: water, acid or acid solution, amine-acid salt solution, humectant and organic co-solvent, amine base, metal corrosion inhibitor, preservative or biocide, solvo-surfactant, soluble azo dye, pigment dispersion, surfactant, and antifoamant. The aqueous inkjet ink composition was mixed for about 2 minutes between ingredient additions, and then it was stirred for 1 hour after the final addition of the surfactant or antifoamant. The aqueous inkjet ink composition was filtered through Pall Corp. 0.2 μm effective pore size ULTIPOR® N66 cartridge filter media at a rate of about 0.5 liter/min/inch (0.2 liter/min/cm) of media. The pigment particles in the resulting aqueous inkjet ink composition had a volume-weighted 50^(th) percentile particle size of 53 nm and 95^(th) percentile particle size of 79 nm, a pH of about 8.6, electrical conductivity of 1.14 mS/cm, dynamic viscosity of 1.60 mPa-sec at 25° C., density of 1.035 g/cm³ at 25° C., and a static surface tension of 37.2 mN/m at 25° C.

TABLE I Functional Ink K-A (E) Component Component (weight %) Vehicle Water 48.8 Pigment Dispersion KD-1 33.8 Binder Polymer P-5 7.2 Dispersion Humectant Glycerol 8.5 Metal Corrosion Inhibitor COBRATEC ® 0.1 TT-50S (PMC Specialties Group, Inc.) Solvo- 2-Phenylethanol 1.5 surfactant (Acros Organics) Surfactant SURFYNOL ® 440 0.2 (Evonik Corp.)

Aqueous Cyan Inkjet Ink Compositions

Aqueous cyan CIJ inkjet ink compositions C-A through C-C were prepared from the pigment dispersion CD-1 by combining the components at the relative proportions shown below in TABLE II in a manner analogous to that described for the aqueous inkjet ink composition K-A. These inkjet ink compositions exhibited physical properties that are reported below in TABLE III.

TABLE II C-A (C) C-B (E) C-C (E) Functional (weight (weight (weight Component Ingredient %) %) %) Vehicle Water 74.1 72.6 72.6 Pigment CD-1 16.7 16.7 16.7 Dispersion Binder P-5 4.0 4.0 4.0 Polymer Dispersion Stabilizer LUVITEC ® 0.5 0.5 0.5 Polymer K 17 (BASF) Humectant Glycerol 4.0 4.0 4.0 Metal COBRATEC ® 0.1 0.1 0.1 Corrosion TT-50S Inhibitor (PMC Specialties Group, Inc.) Solvo- 2-Phenoxyethanol 0.0 1.5 0.0 surfactant (Sigma-Aldrich) Solvo- 2-Phenylethanol 0.0 0.0 1.5 surfactant (Acros Organics) Surfactant SURFYNOL ® 0.1 0.1 0.1 440 (Evonik Corp.) Antifoamant SURFYNOL ® 0.1 0.1 0.1 DF-110D (Evonik Corp.)

TABLE III Properties C-A (C) C-B (E) C-C (E) Particle Size 28 29 30 50^(th) percentile (nm) Particle Size 86 84 81 95^(th) percentile (nm) pH 8.5 8.4 8.4 Conductivity (mS/cm) 1.99 2.05 2.12 Density, 25° C. 1.023 1.024 1.022 (g/cm³) Dynamic Viscosity, 1.34 1.40 1.40 at 25° C. in mPa-sec Static Surface Tension, 37.8 36.7 36.5 at 25° C. in mN/m

Aqueous Magenta Inkjet Ink Compositions

Aqueous magenta CIJ inkjet ink compositions M-A through M-C were prepared from the pigment dispersion MD-1 by combining the components at the relative proportions reported below in TABLE IV in a manner analogous to that described for the aqueous black inkjet ink K-A. These aqueous magenta inkjet ink compositions exhibited physical properties that are reported below in TABLE V.

TABLE IV M-A (C) M-B (E) M-C (E) Functional (weight (weight (weight Component Component %) %) %) Vehicle Water 55.4 53.9 53.9 Pigment MD-1 31.7 31.7 31.7 Dispersion Binder P-5 6.0 6.0 6.0 Polymer Dispersion Humectant Glycerol 6.5 6.5 6.5 Metal COBRATEC ® 0.1 0.1 0.1 Corrosion TT-50S Inhibitor (PMC Specialties Group, Inc.) Solvo- 2-Phenoxyethanol 0.0 1.5 0.0 surfactant (Sigma-Aldrich) Solvo- 2-Phenylethanol 0.0 0.0 1.5 surfactant (Acros Organics) Surfactant SURFYNOL ® 0.22 0.22 0.22 440 (Evonik Corp.) Antifoamant SURFYNOL ® 0.1 0.1 0.1 DF-110D (Evonik Corp.)

TABLE V Properties M-A (C) M-B (E) M-C (E) Particle Size 15 14 15 50^(th) percentile (nm) Particle Size 60 59 57 95^(th) percentile (nm) pH 8.7 8.6 8.6 Conductivity (mS/cm) 1.51 1.50 1.52 Density, 25° C. 1.033 1.035 1.033 (g/cm³) Dynamic Viscosity, 1.74 1.87 1.83 at 25° C. in mPa-sec Static Surface Tension, 38.7 37.1 36.8 at 25° C. in mN/m

Aqueous Yellow Inkjet Ink Composition

Aqueous yellow CIJ inkjet ink composition Y-A (E) was prepared from the pigment dispersion YD-1 by combining the components at the relative proportions reported below in TABLE VI in a manner analogous to that described for the aqueous inkjet ink composition K-A.

TABLE VI Functional Y-A (E) Component Component (weight %) Vehicle Water 60.9 Pigment Dispersion YD-1 25.0 Binder Polymer P-4 4.0 Solution Binder Polymer P-5 3.3 Dispersion Humectant Triethylene glycol 5.0 Metal Corrosion COBRATEC ® 0.1 Inhibitor TT-50S (PMC Specialties Group, Inc.) Solvo- 2-Phenylethanol 1.5 surfactant (Acros Organics) Surfactant SURFYNOL ® 440 0.1 (Evonik Corp.) Antifoamant SURFYNOL ® 0.1 DF-110D (Evonik Corp.)

The pigment particles in this aqueous inkjet ink composition had a volume-weighted 50^(th) percentile particle size of 12 nm and a 95^(th) percentile particle size of 31 nm as characterized by the NANOTRAC® NPA 150 dynamic light scattering particle sizing instrument. It also had a pH of about 8.4, electrical conductivity of 2.26 mS/cm, dynamic viscosity of 1.62 mPa-sec at 25° C., density of 1.020 g/cm³ at 25° C., and a static surface tension of 36.3 mN/m at 25° C.

Continuous Inkjet Printing (CIJ) Replenisher Fluid

CIJ Replenisher Fluids R-A (E) and R-B (E) were prepared by combining the components shown below in TABLE VII at the relative proportions in a manner analogous to that described for aqueous black inkjet ink composition K-A. These aqueous particle-free fluids exhibited physical properties that are shown below in TABLE VIII.

TABLE VII Functional Fluid R-A (E) Fluid R-B (E) Component Component (weight %) (weight %) Vehicle Water 99.0 99.0 Solvo- 2-Phenoxyethanol 1.0 0.0 surfactant (Dow Chemical Co.) Solvo- 2-Phenylethanol 0.0 1.0 surfactant (Acros Organics)

TABLE VIII Properties Fluid R-A (E) Fluid R-B (E) pH 6.5 6.2 Conductivity (mS/cm) 0.001 0.001 Density, at 25° C. in g/cm³ 0.999 0.998 Dynamic Viscosity, 1.018 0.923 at 25° C. in mPa-sec Static Surface Tension, 46.9 46.1 at 25° C. in mN/m

Continuous Inkjet Printing (CIJ) Printhead Cleaner and Storage Fluid

CIJ Printhead Cleaner and Storage Fluids S-A (C) and S-B (E) were prepared by combining the components below in TABLE IX at the relative proportions in a manner analogous to that described for the aqueous black inkjet ink composition K-A. These aqueous particle-free fluids exhibited physical properties that are shown below in TABLE X.

TABLE IX Functional Fluid S-A (C) Fluid S-B (E) Component Component (weight %) (weight %) Vehicle Water 83.7 81.3 Base N,N-Dimethyl- 0.1 0.5 aminoethanol Acid Acetic acid, 25% 0.1 1.2 Humectant Triethylene glycol 10.0 10.0 Metal Corrosion COBRATEC ® 0.1 0.1 Inhibitor TT-50S (PMC Specialties Group, Inc.) Solvo- Diethylene glycol 5.0 5.0 surfactant monobutyl ether Solvo- 2-Phenoxyethanol 0.0 1.0 surfactant (Dow Chemical Co.) Surfactant ZETASPERSE ® 1.0 1.0 1600 (Evonik Corp.)

TABLE X Properties Fluid S-A (C) Fluid S-B (E) pH 9.2 8.6 Conductivity (mS/cm) 0.44 2.12 Density, at 25° C. in 1.015 1.017 g/cm³ Dynamic Viscosity 1.64 1.65 at 25° C. in mPa-sec Static Surface Tension, 28.3 29.3 at 25° C. in mN/m

Microbiological Growth Susceptibility Testing:

The aqueous inkjet ink compositions described above were subjected to a Ten Challenge preservative efficacy test employing cultured strains of particular microorganisms. The test organisms were individually cultured to known concentrations of colony forming units (CFU) and they were then mixed together and inoculated into 50 grams of the aqueous inkjet ink composition test sample. At specified time intervals, the inoculated sample was tested. The surviving population of inoculum was evaluated by streaking a Trypticase Soy Agar plate with 10 μl of well-mixed sample and incubating. The presence of microorganisms was documented using the growth ratings shown in the following TABLE XI. Each sample was re-inoculated and evaluated up to ten times over the course of the study, while any changes in the physical appearance of the sample were observed.

TABLE XI Microorganism Growth Rating Colonies 1 0 2 1 to 4 3 5 to 10 4 11 to 25 5 26 to 50 6 51 to 100 7 101 to 200 8 201 to 300 9 TNTC* 10 Confluent growth *TNTC means “too numerous to count”

The following TABLE XII reports the 10 Challenge growth rating test results for the previously described aqueous inkjet ink compositions (“ink”), wherein the initial baseline growth rating for microorganisms in the as-received composition was determined to be “1.” The column identified with “R—OH” denotes the absence or presence of solvo-surfactant 2-phenoxyethanol (PhE) or 2-phenylethanol (PEA) in the sample.

TABLE XII Entry Ink R-OH 1 2 3 4 5 6 7 8 9 10 1 (C) C-A — 9 9 6 9 6 9 9 9 9 6 2 (E) C-B PhE 1 1 1 1 1 2 1 2 1 1 3 (E) C-C PEA 1 1 1 1 1 1 1 1 1 1 4 (C) M-A — 1 4 2 2 5 4 5 4 2 2 5 (E) M-B PhE 1 1 1 1 1 1 1 1 1 1 6 (E) M-C PEA 1 1 1 1 1 1 1 1 1 1 7 (E) Y-A PEA 1 1 1 1 1 1 1 1 1 1 8 (E) K-A PEA 1 1 1 1 1 1 1 1 1 1

The results shown in TABLE XII indicate that considerable microorganism growth (high colony counts) was observed in the comparative composition (“ink”) C-A (C) and M-A (C) after the first few inoculations. The data in TABLE XII also reveal that the presence of 2-phenoxyethanol (PhE) in inventive compositions (“Inks”) C-B (E) and M-B (E) or 2-phenylethanol (PEA) in inventive compositions (“Inks”) C-C (E), M-C (E), Y-A (E), and K-A (E) resulted in minimal or undetectable microbial growth through all ten challenges.

In an identical manner to the aqueous inkjet ink compositions, the aqueous particle-free fluids were subjected to a Ten Challenge preservative efficacy test employing the same cultured strains of particular microorganisms. The following TABLE XIII shows the 10 Challenge growth rating test results for those fluids, wherein the initial baseline growth rating for microorganisms in the as-received fluids was determined to be “1.” The column identified with “R—OH” denotes the absence or presence of solvo-surfactant 2-phenoxyethanol (PhE) or 2-phenylethanol (PEA) in the fluid.

TABLE XIII Entry Fluid R-OH 1 2 3 4 5 6 7 8 9 10 1 (E) R-A PhE 1 2 4 7 5 2 1 2 2 4 2 (E) R-B PEA 2 1 1 1 1 6 6 6 4 6 3 (C) S-A — 1 1 1 1 1 1 3 1 1 1 4 (E) S-B PhE 1 1 1 1 1 1 1 1 1 1

The results reported in TABLE XIII show that significant microorganism growth (increased colony counts) was observed in the inventive Replenisher Fluids R-A (E) and R-B (E) after the first few inoculations, but the result is expected to be better than a replenisher fluid comprised purely of water without any solvo-surfactant. The results in TABLE XIII also show that virtually no microorganism growth was observed in the comparative Printhead Cleaning and Storage Fluid S-A (C), which contained solvo-surfactant diethylene glycol monobutyl ether as a dried ink cleaning agent; only challenge No. 7 produced a significant growth rating of 3. However, the presence of 2-phenoxyethanol (PhE) in inventive Printhead Cleaning and Storage Fluid S-B (E) produced undetectable microbial growth through all ten challenges.

It will be appreciated that embodiments of aqueous inkjet ink compositions and aqueous particle-free fluids according to the present invention can be suitably resistant to growth of contaminating microorganisms without containing any substantial amounts of, that is, being substantially free of, commonly used preservatives such as isothiazolinone compounds and formaldehyde releasing compounds (for example, 2-bromo-2-nitropropane-1, 3-diol and hexamethylenetetramine chloroallyl chloride) and biocides currently listed in the List of Approved Active Substances (Article 95 List) published by the European Chemical Agency (ECHA) (an agency of the European Union).

Continuous Inkjet Printing of Aqueous Inkjet Ink Compositions:

In a representative procedure, the ink reservoir of a continuous inkjet printing test stand fixture was charged with inventive aqueous cyan inkjet ink composition C-C (E). Repetitive cycles of draining, flushing, and filling the ink reservoir and fluid lines with C-C (E) were carried out to ensure that the new composition was not contaminated by the prior ink in the apparatus. The fixture consisted of the following elements: (1) a fluid system capable of (a) pressurizing the composition (“ink”) in excess of 60 psid (0.41 MPa) thereby producing ink volumetric flow rates of up to about 2 liters/min; (b) delivering pressurized ink to a continuous inkjet printhead drop generator; (c) returning unprinted ink under vacuum to the fluid system ink reservoir; (d) detecting the ink reservoir ink concentration by electrical resistivity measurement and replenishing the ink with inventive Replenisher Fluid R-B (E) if concentrated by water evaporation, and adding more aqueous inkjet ink composition C-C (E) to the ink reservoir instead if it was depleted by use in printing but was at the correct concentration; (e) providing the printhead with inventive Printhead Cleaning and Storage Fluid S-B (E) to flush the nozzles and duct systems in order to restore accurate printing after fouling by dried ink build-up, and to shut down the system for safe storage over significant time durations; (2) a vacuum drum capable of supporting a sheet of porous media (for example, uncoated free sheet paper) or non-porous media (for example, coated or uncoated polymer film) and spinning it continuously at precise speeds synchronized with a control unit to simulate web transport of the printing substrate in roll form; (3) a continuous inkjet printhead PIC box assembly including (a) a KODAK PROSPER Press Jetting Module with a MEMS silicon-based drop generator to form drops of ink and a Coanda gutter to catch non-printing drops when the printer is not printing an image file or when it is not printing a given pixel even if it is printing an image file; (b) a non-printing drop deflection apparatus creating a zone intersecting the drop curtain provided by positive and negative air duct assemblies to direct those drops to the Coanda gutter, and (c) an ink return line to the ink reservoir, and (4) a print controller that (a) controls the printing drum speed and synchronizes the drum location in accord with the data feed to the jetting module and also (b) transmits electrical signals to the jetting module CMOS circuitry that renders a raster processed image into pixel by pixel ink stream stimulation instructions using nozzle plate heater pulse patterns by optimized waveforms to generate non-printing catch drops and printing drops of ink delivered at the printing substrate surface pixel locations, as required.

The fluid system used a Micropump Inc. MICROPUMP® series GJ-N23DB380A gear pump to deliver the ink through a Pall Corp. Disposable Filter Assembly capsule filter, DFA4201ZU0045, containing 0.45 μm nominal effective pore size ULTIPOR® GF-HV glass fiber media at about 65 psid (0.45 MPa) pressure drop at the nozzle plate, which generated a uniform drop velocity of about 20 m/s. The fluid system gear pump speed setting was continually adjusted to provide and maintain constant fluid pressure at the jetting module to uniformly produce the desired drop velocity as per the system specification. The required system parameter settings for proper jetting and accurate ink replenishment were determined and recorded to a computer file termed an “inkdex” to enable printing on other systems, such as a web press fitted two-up with production KODAK® PROSPER S10 Imprinting Systems. The deflected non-printing ink drops were caught on a Coanda gutter and returned to the fluid system ink tank under vacuum. Sustained operation of the printer in catch mode of the non-printing drops resulted in gradual evaporation of the aqueous ink solvent vehicle. Ink concentration was maintained to within about 5% of the original ink concentration by addition of the aqueous pigment-free Replenisher Fluid to the ink, if the latter became more than about 5% concentrated based on an ink electrical resistivity determination. Test targets were raster image processed to produce digital printing signal instructions for each pixel location at the appropriate transport speed of the test substrate at 600×600 pixels per inch (ppi) (236×236 pixels per centimeter (ppcm)) addressability for speeds up to about 1,000 FPM. NewPage STERLING® Ultra Gloss paper and/or an uncoated, untreated free sheet paper (for example, International Paper 20-lb (75 g/m²) DATASPEED® Laser MOCR) was loaded on to the constant speed, rotating drum, which was synchronized with the print data controller. Various test images were printed at different substrate transport speeds that profiled system functional printing speed capability using a 600-nozzles per inch PROSPER Press Jetting Module in a near-production print-head assembly configuration, which produced a 4.25-inch (10.8 cm) jet curtain print swath. Operational stability and start-up robustness (“runnability”) were also probed by printing into a catch pan and assessing time-dependent jet straightness and print raggedness for extended periods, and by conducting multiple shut-down and start-up sequences to assess the time to achieve the first acceptable print and any required interventions [service clean (cross-flush with ink), duct clean (flush with storage fluid), and nozzle plate wipe]. Aqueous inkjet ink composition C-C (E) was found to be runnable and was loaded onto a two-up web press fitted with production KODAK PROSPER S10 Imprinting Systems for long-term testing.

The present invention has been demonstrated above for use in a continuous ink jet printer system that employs a gas flow drop deflection mechanism, thermal drop stimulation devices, and nozzle plates fabricated out of silicon. However, the present invention can also be employed in continuous ink jet printer systems that use electrostatic drop deflection mechanisms, pressure modulation or vibrating body stimulation devices, and nozzles plates fabricated out of other types of materials. Electrostatic deflection can be of the type that includes separate drop charging and drop deflection electrodes or can be of the type that incorporates both functions in a single electrode.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be obtained within the spirit and scope of the invention. 

1. A fluid set comprising two or more aqueous particle-free fluids, each of which two or more aqueous particle-free fluids independently has a dynamic viscosity of less than or equal to 5 centipoise (5 mPa-sec) at 25° C., and each of which two or more aqueous particle-free fluids independently consists essentially of: (b) a composition consisting of one or more compounds represented by the following Structure (I): HO—CH₂—CH₂—R  (I) wherein R is a substituted or unsubstituted phenyl group or a substituted or unsubstituted phenoxy group, in a total amount of at least 0.5 weight % and up to and including 2 weight %, based on the total weight of the aqueous inkjet ink composition.
 2. The fluid set of claim 1, wherein each of the two or more aqueous particle-free fluids further contains one or more promoters for the one or more Structure (I) compounds of the (b) composition, each of which promoters is an alkane diol having a least 7 carbon atoms and up to and including 12 carbon atoms, in an amount of less than or equal to 1.5 weight %, based on the total weight of the aqueous inkjet ink composition.
 3. The fluid set of claim 2 wherein the one or more promoters comprises 1,2-octanediol.
 4. The fluid set of claim 1, wherein R is either an unsubstituted phenyl group or an unsubstituted phenoxy group.
 5. The fluid set of claim 1, wherein at least one of the two or more aqueous particle-free fluids further comprises at least one water-soluble or water-dispersible polymeric component in an amount of up to and including 20 weight %, based on the total weight of the at least one aqueous particle-free fluid, which at least one water-soluble or water-dispersible polymeric component is an acrylic polymer, a styrene-acrylic polymer, water-soluble polymer having poly(ethylene oxide) segments, water-dispersible polyurethane, or two or more of such materials.
 6. The fluid set of claim 1, wherein at least one of the two or more aqueous particle-free fluids has a pH of at least 8 and up to and including
 11. 7. The fluid set of claim 1, wherein at least one of the two or more aqueous particle-free fluids comprises an anionic, nonionic, cationic, or amphoteric surfactant in an amount of least 0.01 weight % and up to and including 5 weight %, based on the total weight of the at least one aqueous particle-free fluid.
 8. The fluid set of claim 1, wherein at least one of the two or more aqueous particle-free fluids comprises a (d) supplemental antimicrobial agent that is different from the (b) composition.
 9. The fluid set of claim 8, wherein the (d) supplemental antimicrobial agent is iodopropynyl butyl carbamate, piroctone olamine, 2,4-dichlorobenzyl alcohol, boric acid or a metal ion salt derived from boric acid, or a combination of these compounds.
 10. The fluid set of claim 1, wherein at least one of the two or more of the aqueous particle-free fluids further comprises a solvo-surfactant in an amount of at least 0.1 weight % and up to and including 20 weight %, based on the total weight of the at least one aqueous particle-free fluid.
 11. The fluid set of claim 10, wherein the solvo-surfactant is an asymmetric polyhydric alcohol or mono-alkyl ether derived from a polyhydric alcohol.
 12. The fluid set of claim 10, wherein the solvo-surfactant is a C₁ to C₄ mono-alkyl ethyl derived from a polyhydric alcohol.
 13. The fluid set of claim 10, wherein the solvo-surfactant is present in an amount of at least 3 weight % and up to and including 6 weight %, based on the total weight of the at least one aqueous particle-free fluid.
 14. The fluid set of claim 10, wherein the at least one of the two or more aqueous particle-free fluids further comprises a surfactant different from the solvo-surfactant, which surfactant is present in an amount of at least 0.01 weight % and up to 10 weight %, based on the total weight of the at least one aqueous particle-free fluid.
 15. The fluid set of claim 1, wherein at least one of the two or more aqueous particle-free fluids has a pH of at least 10 and up to and including 11, and further comprises an organic base.
 16. The fluid set of claim 15, wherein the organic base is an alkanolamine. 